Ore milling and grinding is an important process step in the mining industry. SAG mills and ball mills are generally used for this purpose. In both cases the mills are tube mills or drum mills which, considered in simplified terms, consist of a rotating cylinder (drum) which is filled with the ore that is to be ground. The rotation of the drum causes material that is to be crushed to move upward in the mill and subsequently to fall onto the remaining material on the floor of the mill due to the force of gravity. The impact of the particles as well as the attrition within the circulating charge lead to a disintegration of for example the ore. In many mill systems steel balls are also added to the material in the milling plant in order to improve milling efficiency.
For the purpose of optimal control it is important to be able to measure characteristic variables of the comminution process such as for example the masses of rock and water inside the mill, the material flows into and out of the mill, or the comminution or reduction rate for grinding coarse rock particles into fine rock particles. Due to the harsh conditions within the mill it is generally not easy to conduct direct measurements inside the mill. Furthermore, the large flows of material, in particular in the order of magnitude of thousands of tonnes per hour, cannot be analyzed in detail outside of the mill.
A plurality of (indirect) measuring instruments exist for capturing process variables of milling systems. The input/output mass feeds, the mill weight, the power draw, and the speed of a reduction mill are measured in the conventional manner. Said measurements are typically incorporated in a macroscopic state-space model which takes into account the masses of fractions having different sizes within the mill by means of mass balance equations ([1] [2]).